1. Field of the Invention
This invention relates to the construction of air delivery systems in clean rooms and, more particularly, to a modular clean room plenum for semiconductor manufacturing, aerospace, pharmaceutical and medical clean rooms and other applications where large volumes of particulate free, temperature and humidity controlled vertical laminar airflow are required.
2. Description of Related Art
Clean room air delivery systems are generally designed to filter out dirt and dust particles of a very small size, correct the humidity and temperature of the air, and supply that air into the clean room in a laminar airflow pattern. The laminar airflow may be either vertically downward from the ceiling to the floor, horizontally from one side of the clean room space to the other, or horizontally across the clean room work surface, and then downward to the floor. The vertically downward airflow direction is the most common in the industry.
The volume of air delivery to the clean room ranges from approximately 30 cubic feet per minute to 120 cubic feet per minute per square foot of clean room floor space. This volume compares to 1.0 to 1.5 cubic feet per minute per square foot of floor space in a typical office building. Such clean room air delivery systems are often used in semiconductor manufacturing clean rooms, but have numerous applications where a particulate-free, temperature and humidity controlled environment is required.
The design and construction process for structures built as clean rooms is typically both lengthy and costly. FIG. 1 illustrates a conventional clean room and air barrier arrangement. Based on existing design principles, the normal sequence is to construct the building's foundations and shell 11, including an extensive primary support structure 13 spanning the width and length of the clean room area using a minimum of intermediate support columns 12. Primary support structure 13 may be constructed of steel trusses, steel space frames, or various types of concrete. A roofing system added to the top of the primary support structure 13 forms a primary air barrier 3 to contain air within the building.
Next, secondary support structure 37 is attached to and supported by the bottom of the primary support structure 13. Secondary support structure 37 will support a secondary air barrier 35 covering the entire clean room area. The purpose of secondary air barrier 35 is to separate the "conditioned" supply air from the "dirty" return air. The "conditioned" supply air becomes "dirty" as it passes through the clean room space 17 and picks up heat, humidity, and dirt particles from persons, products, and machinery in the clean room space 17. Depending upon the particular design of the clean room 17, the "conditioned" supply air may be above the secondary air barrier 35 with the "dirty" return air below the barrier 35, or the "dirty" return air may be above the secondary air barrier 35 and the "conditioned" supply air below.
Secondary air barrier 35 must be sufficiently strong to support the weight of workers who may have to enter the space above secondary air barrier 35 to conduct maintenance or modifications, and to support the entire underlying ceiling grid 39 and all of its components.
Following installation of the secondary air barrier 35, a tertiary support system 45 is installed on the underside of the secondary air barrier 35 to support the ceiling grid 39 and its components. The secondary support structure 37 may also be required to support an automated material handling system 63 (a means of distributing product throughout the clean room) or other production equipment. The supports for such a material handling system 63 must penetrate the secondary air barrier 35 and are a source of air leaks, as well as being difficult to construct. The ceiling grid 39 which forms the tertiary air barrier comprises a sealed structural support system that may contain, but is not limited to, air filters, return air grilles, blank panels, and lights. The ceiling grid 39 also provides support for the fire sprinkler system. A piping system is added to the assembled ceiling grid 39, a sprinkler main (not shown) is connected to the piping system, sprigs are installed, and sprinkler heads are connected to ceiling grid 39. Electrical wiring and light fixtures are also connected to the ceiling grid 39.
An exemplary ceiling grid 39 is described in U.S. Pat. No. 5,613,759 to Ludwig, Spradling, and Benson. As described in Ludwig et al., the ceiling grid 39 has a grid of interconnected rails in a rectangular pattern with openings between the rails generally 2'.times.4' in dimension. These rails have moat-like channels on each side, which form a continuous moat around all sides of the rectangular openings. All of the rectangular openings will be in-filled with, e.g., high efficiency particulate filters (filters may be of any type well known in the art, such as a HEPA or ULPA filters, similar to those manufactured by, e.g., Flanders or Filtra), blank panels, lights, sprinkler head panels, and return air grills. The items installed in the ceiling grid 39 have downwardly depending flanges around their peripheral edges that fit into the moat-like channels of the grid 39.
After each rectangular opening is filled, a gel sealant, e.g., BioMed 246 manufactured by Formula Brand Coatings & Products, is poured into the moat-like channels to seal the entire ceiling grid 39. This sealed ceiling grid 39 forms the tertiary air barrier. Alternatively, other forms of sealants can be used to seal the ceiling grid. In addition, other types of ceiling grids 39 use T-shaped interconnecting rails and filters, blank panels, lights, sprinkler head panels, and return air grills with flat bottoms rather than downwardly depending flanges. These various panels are sealed into the grid system 39 using various forms of gaskets to prevent air leakage. Before the installation of the gel-sealant into the channels of ceiling grid 39, the interior of building shell 11 must receive a thorough cleaning to remove dirt particles introduced into the space during construction.
After the ceiling grid 39 is installed, transfer air ducts 9 are installed extending from the secondary air barrier 35 to the ceiling grid 39. In most clean room installations, there is an array of transfer air ducts 9. However, for clarity, FIG. 1 illustrates only one transfer air duct 9. The transfer air ducts 9 may carry either "conditioned" supply air or "dirty" return air depending upon the air flow pattern of the design. If the "conditioned" supply air is above the secondary air barrier 35, transfer air ducts 9 with balancing dampers 91 and flex connections 31 are installed from the secondary air barrier 35 down to each of the filters in the ceiling grid 39. These transfer air ducts 9 will deliver "conditioned" supply air through the filters and into the clean room 17. In this case, "dirty" return air passes through the return air grilles directly into the "dirty" return air plenum 41 below the secondary air barrier 35. This embodiment is illustrated in FIG. 1.
If the "conditioned" supply air is located below the secondary air barrier 35, the "conditioned" supply air passes directly through the filters into the clean room space 17. The "dirty" return air must then be ducted from the return air grills 5 (FIG. 3) in the ceiling 39 up through the secondary air barrier 35 into the "dirty" return air plenum 41.
The "dirty" return air is taken through a recirculation air handling unit 21, returned as "conditioned" supply air and delivered through the filters to the clean room space 17. Recirculation air handling units 21 of this type are typically located outside the clean room space 17. Alternatively, "conditioned" supply air may be circulated through a fan unit (not shown) located above the ceiling grid 39 and below the secondary air barrier 35. These units are generally referred to as a fan filter units (FFU).
The operation of the arrangement shown in FIG. 1 is as follows. Recirculation air handler (RAH) unit 21 takes air from return air plenum 41 in the direction indicated by RAH inlet airflow arrow 27. The RAH unit 21 corrects the temperature and humidity of the air, and supplies it to air supply plenum 40 in the direction of conditioned supply arrow 15 at an increased air pressure. The supply air travels down through an array of transfer air ducts 9, dampers 91, and air filters 47, into clean room 17 in a laminar airflow pattern. The laminar airflow travels downward as indicated by arrow 29 towards raised floor 18. The air travels through raised floor 18 via air holes provided in the floor, passes through subfloor region 19 to return air chase 43, up through ceiling grid 39 in the direction of arrow 16 and back into return air plenum 41. In return air plenum 41, return air from the clean room 17 is mixed with air from outside the building, provided by makeup air unit 23.
Makeup air unit 23 takes outside air, adjusts it for interior temperature and humidity requirements, filters it, and supplies the air through ductwork 51 to return air plenum 41 along the path illustrated by MAU airflow arrow 25. The new air from makeup air unit 23 mixes with the return air from the clean room 17 and is processed by recirculation air handler 21 to be supplied back to clean room 17, as described above.
Design of the clean room 17 requires very specific and carefully controlled air velocities in a vertical airflow pattern. For the clean room 17 to be certified, the air velocity must be within certain limits of the design velocities, usually 5% to 10% of design. The air velocity can be set and controlled by adjusting balancing dampers 91 located in the air supply to each of the air filters 47 in the ceiling grid 39, and by adjusting the settings on the recirculation air handling unit 21. In clean rooms 17, the balancing dampers 91 may be located at the bottom end of the transfer air ducts 9 above the ceiling grid 39 and below the secondary air barrier 35.
Balancing typically requires a two-step process because the balancing damper 91 is in an inaccessible location when the ceiling grid 39 is completed. A preliminary balance is achieved by adjusting the balancing dampers 91 before the ceiling grid 39 is completed, followed by a final balance after completion of ceiling grid 39. This process is time-consuming and interferes with the construction process, and can lead to certification delays if the preliminary balance was not accurate. To correct an inaccurate damper balance, the ceiling grid 39 will have to be opened and the balancing damper 91 adjusted properly. If the airflow requirements of the clean room 17 change, it usually is necessary to shut down the manufacturing operation so the ceiling grid 39 may be opened and the balancing dampers 91 accessed.
Buildings housing clean rooms 17 typically require the construction to proceed in a set sequence. First the foundations are constructed, then the primary structural support 13, followed by the roofing system. With the roofing system complete, construction of the clean room air distribution system may begin. First, the secondary support system 37 is installed, followed in sequence by the secondary air barrier 35, the tertiary support system 45 and the ceiling grid system 39. The final stage of the construction is the installation of the transfer air ducts 9, the fire sprinkler system, the filters, blank panels, sprinkler head panels and return air grilles. Preliminary balancing of the dampers 91 must take place before the ceiling grid 39 is completed, followed by final balance after completion of the ceiling grid 39.
Construction of these systems is difficult and time-consuming due the necessity of working high above the clean room floor. This arrangement requires a means to lift the workers up to the construction level, or that workers stand on the steel truss sections, either of which increases costs, construction time, and the danger of injury from falls. In addition, much of the construction that takes place must be done in a manner such that the amount of contaminants brought into the clean room space 17 is held to a minimum. This requires that construction personnel practice clean room protocol, including cleaning tools and equipment before being brought into the clean room, wearing protective clothing, and executing the construction using pre-approved clean room techniques. The productivity level of all work conducted in clean room 17 is measurably lower than the same work conducted outside clean room 17.
Due to building code requirements, the space between the ceiling grid system 39 and the secondary air barrier 35 and the space between the secondary air barrier 35 and the roofing system must be protected from the effects of fire. Although the space above the secondary air barrier 35 is typically unoccupied, because it supports a major portion of the building, it therefore must be fire protected to meet safety codes. This fire protection is typically applied before the secondary air barrier 35 is installed. The clean room space 17 below the ceiling grid system 39 is generally considered by building codes to be an "occupied space", therefore requiring application of stricter fire protection rules. The return air plenum area is generally protected by fire sprinkler risers, called "sprigs", connected to the conventional fire sprinkler system serving the clean room space 17. These sprigs are usually installed after the ceiling grid system is in place and before the filters, blank panels, sprinkler head panels and return air grilles are installed.
In most clean room environments, the manufacturing process requires that the clean room space 17 be divided into separate and distinct zones, requiring a complete separation of the air streams from the primary air barrier 3 through the clean room subfloor 19 to avoid cross-contamination of the air between various processes. Also, many buildings are constructed larger than current manufacturing space requirements, and the "future growth" areas must be separated from the utilized clean room space. In both cases, the separation is typically achieved using vertical barriers, referred to as "demising walls".
Demising walls are typically connected to the building structural system for support, and are constructed similar to a standard wall, using sheet rock, sheet metal or special clean room panels for the wall surface. This type of vertical barrier is labor intensive to construct and difficult to move if the demands of the manufacturing process require a change. Using existing clean room construction techniques, adjustments to the demising walls will incur substantial costs and usually disturb the existing manufacturing process.
In the semiconductor manufacturing industry, the total construction time for a facility has significant cost ramifications. It is not uncommon for semiconductor fabrication clean rooms to output over a million dollars worth of products per hour. Accordingly, any decrease in construction time accelerates the schedule for producing wafers, which can generate substantial amounts of money. Thus, in addition to the overall need for cutting construction costs, there is a significant economic incentive for streamlining and shortening the construction process for semiconductor clean room facilities.
Accordingly, it is clear there is a need for an improved clean room plenum system design that lowers construction costs, decreases construction timelines, easily adapts to changing manufacturing space requirements, and can be constructed with increased safety. The plenum system based upon such a design should be easily modifiable with respect to area partitioning, clean room expansion, filter locations, blank panel locations, "dirty" air return locations, lighting locations and fire sprinkler layout. The plenum system should afford the ability to hang automatic material handling systems (AMHS) and other production equipment from the ceiling grid without having to penetrate the secondary air barrier to attach to the secondary support structure. The plenum system should reduce the time required to achieve the critical air balance requirements and make re-balancing relatively easy. The plenum system should also greatly reduce the hazards of working high above the clean room floor during installation and modification.